Development of cancer is typically a multi-step process that depends on many genetic and epigenetic alterations in the tumor cells. In addition, cancer progression is modulated by interactions between the tumor cells and extracellular growth signals. This application investigates how genetic alterations and specific extracellular growth signals interact to modulate tumor progression. Mammalian cells that acquire a single activated oncogene frequently enter a state of irreversible proliferation arrest, called senescence. This "oncogene-induced senescence" acts an important tumor suppression process, by arresting proliferation of nascent tumor cells and therefore preventing their progression along a tumorigenic pathway. Formation of several cancers is suppressed by this mechanism, including human melanomas, human prostate cancer, T-cell lymphomas in mice and, likely, colon cancers. Most strikingly, benign human nevi (moles) are pre-neoplastic lesions comprised of melanocytes, made senescent by oncogenic activation of the Ras-signaling pathway. In mammalian tissues, the canonical Wnt-signaling pathway typically maintains cell proliferation, for example of adult tissue stem cells. This pathway is activated by extracellular Wnt ligands that trigger a cascade of cytoplasmic and nuclear events, culminating in expression of proliferative genes. Recently, we found that Wnt-signaling antagonizes oncogene-induced senescence, and vice versa. This points to a previously unappreciated cross-talk between these two very important cell proliferation- control processes, both of great significance to cancer. In particular, these results suggest that extracellular growth signals, such as canonical Wnt ligands, can modulate cancer progression by affecting the efficiency of oncogene-induced senescence and its resultant tumor suppression activity. We will test these ideas through the following Specific Aims: Specific Aim 1. Define how Wnt-signaling suppresses oncogene-induced senescence. Specific Aim 2. Investigate whether Wnt-signaling drives melanoma formation by inhibiting oncogene- induced senescence in melanocytes. Specific Aim 3. Investigate whether Wnt-signaling drives colon cancer by inhibiting oncogene-induced senescence in colonic epithelial cells. PUBLIC HEALTH RELEVANCE: Recently, we found that Wnt signaling (tumor-promoting) suppresses oncogene-induced senescence (tumor-suppressing). We will test whether this new-found functional interaction contributes to tumor progression in vivo. Specifically, we will focus on melanoma and colon cancer, two cancers which between them kill about 60,000 people a year in the United States.